The present invention relates to a switching regulator control circuit for a PFM control circuit having a stable duty cycle and noise immunity when an output voltage closely approaches a set voltage.
The basic operation of a switching regulator control circuit is as follows. An output voltage is fed back and compared with a reference voltage by an output voltage control comparator. When the output voltage becomes smaller than a set voltage, the comparator is operated to send an oscillation start signal to an oscillating circuit. Then, the oscillating circuit outputs a switching pulse, so that an output transistor is turned ON or OFF to thereby adjust the output voltage.
FIGS. 5A and 5B show an example of a step-up DC-DC converter to which a PFM control switching regulator is applied.
In the PFM control switching regulator, an ON period of an output transistor is always constant. Thus, an OFF period is adjusted so that an output voltage is stably kept to a constant value.
FIG. 2 shows a conventional switching regulator control circuit for a PFM control. As shown in FIG. 2, the output of a reference voltage circuit 11 and that of voltage dividing circuits 12 and 14 are connected with the input of an output voltage control comparator 14. The output of the output voltage control comparator 14 is connected with the input of a ring oscillator circuit 15. The ring oscillator circuit 15 outputs a signal for controlling a transistor that controls an output voltage of a DC-DC converter.
For a PFM switching regulator 10, the switching regulator control circuits for PFM control as shown in FIG. 2 are widely used. A switching pulse output by the ring oscillator circuit 15 is output by controlling an output signal of the above output voltage control comparator 14.
In FIG. 2, when the output signal, Vosc_EN, of the comparator 14 is xe2x80x9cHxe2x80x9d, an oscillating circuit of the ring oscillator circuit 15 is being operated, so that a switching pulse is outputted to EXT. On the other hand when Vosc_EN is xe2x80x9cLxe2x80x9d, the ring oscillator circuit 15 is stopped, so that EXT becomes xe2x80x9cLxe2x80x9d.
When the Vosc_EN signal is changed from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d, EXT immediately becomes from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d. Hereinafter, oscillation is continued. When the Vosc_EN signal is changed from xe2x80x9cHxe2x80x9d to xe2x80x9cLxe2x80x9d, EXT immediately becomes xe2x80x9cLxe2x80x9d.
However, when the output voltage closely approaches a set voltage, chattering of the Vosc_EN output signal of the above comparator 14 is frequently caused due to the influence of noise. Thus, there may arise such an abnormal duty problem that an ON period shorter than a general switching ON period is successively generated in accordance with the chattering.
When an abnormal duty is caused, an OFF period becomes shorter as an output ON period shortens. Thus, a consumption current of the switching regulator control circuit is increased, so that efficiency in the case of a light load is greatly influenced thereby.
In order to solve the above-mentioned problem, there is also given a method for providing hysteresis in a subsequent stage of the output voltage control comparator 14 to suppress chattering of the output signal of the comparator 14. However, in such a method, a new problem is caused for the reason such as reduction in yield due to variation in manufacturing an integrated circuit. Therefore, a more simplified and effective solving method is required.
An object of the present invention is therefore to provide a switching regulator control circuit for providing a PFM control with a stable duty cycle and noise immunity, having a circuit structure which is simplified and effective and in which the difficulty in manufacturing an integrated circuit is eliminated as compared with a conventional hysteresis circuit system, using a simple logic structure which is devised in order to solve the above-mentioned conventional problems.
According to the present invention, a simple logic structure is used, a feedback signal for turning ON an output transistor is fed back, and then a logic signal between the feedback signal and an output signal of an output voltage control comparator is input to a ring oscillator circuit. Thus, even in the case of a frequent variation in the output signal of the comparator, which is liable to occur at a time when an output voltage closely approaches a set voltage, a determined ON period for the output transistor is ensured in the ring oscillator circuit, thereby solving the foregoing problems.